Such a “hands-free” device for a motor vehicle is described by way of example in WO 2008/049982 A1 (Parrot SA) which describes various types of echo cancellation and suppression processing, noise reduction processing, etc., as applied to a signal picked up by a microphone that picks up the acoustic signal from the near speaker.
The echo considered herein—to be distinguished from the “line echo” that arises solely within the transmission path, and for which various filtering methods are known—is the acoustic echo as picked up by the microphone and that is due to the phenomenon of reverberation in the environment of the speaker, typically the room or the vehicle cabin occupied by the speaker, and also to direct acoustic coupling between the loudspeaker and the microphone.
A microphone picks up not only the voice of the near speaker, but also the surrounding noise and, above all, the acoustic echo coming from the sound reproduced by the loudspeaker. The acoustic echo constitutes a major disturbing element of the device that can often be so great as to make the speech of the near speaker (the speaker whose speech is lost in the acoustic echo) incomprehensible for the remote speaker (the speaker at the other end of the transmission path of the telephone signal).
This effect is particularly marked when the microphone and the loudspeaker are close together, and the acoustic power delivered by the loudspeaker is high—as is very often true of systems on board motor vehicles, where the sound level from the loudspeaker is relatively high in order to cover surrounding noise.
Furthermore, numerous “hands-free” devices are implemented in the form of appliances that are self-contained, and removable, comprising a single housing containing both the microphone and the loudspeaker together with control buttons: the proximity (a few centimeters) between the loudspeaker and the microphone then gives rise to a considerable level of acoustic echo, typically of the order of twenty times greater than the speech signal produced by the near speaker.
This effect is manifest essentially during so-called “double talk” situations, i.e. when both speakers are speaking at the same time, because when the remote speaker is speaking the level of the echo that is produced is considerably greater than the mean level of the speech from the near speaker.
Unfortunately, these stages in a call are important since they enhance interactivity between the speakers, and it is important to conserve them (i.e. to maintain so-called “full duplex” communication, as contrasted to alternating or “half-duplex” communication, in which, while one of the speakers is speaking, the other speaker is prevented from intervening.
This double talk situation is very critical for echo cancellation processing (also known as acoustic echo cancellation (AEC)), since it is necessary to estimate dynamically the component that is associated with the acoustic echo and to subtract this estimate from the overall signal picked up by the microphone, but without that degrading the component that is associated with the speech from the near speaker.
In addition, conventional processing in the presence of echo makes use not only of acoustic echo cancellation, but also of post-processing referred to as echo suppression, which applies varying gain to the signal for the purpose of attenuating the residual echo, but does so overall without distinguishing between the residual echo and useful speech, if any is present. As a result, during a period of double talk, this time-varying gain control gives rise simultaneously to a significant degradation in the useful speech.
Various algorithms exist for detecting double talk that make it possible to detect such a situation and to adapt the processing for echo cancellation and for suppressing the residual echo.
Thus, US 2008/0101622 A1 describes echo cancellation processing that implements one or two double talk detectors (DTDs). When two DTDs are used in parallel, an evaluator circuit is provided that delivers a status flag (converged/not converged) concerning the adaptive filter used for the echo cancellation processing. This evaluator circuit is used to control a switch that enables one of the DTDs to take the place of the other once filtering has converged. That flag nevertheless makes no contribution to determining the presence or absence of double talk by either of the DTDs. The convergence status of the adaptive filter is thus without effect on the decision taken by one or the other of the DTDs about the existence or absence of a double talk situation; this status is used only downstream from the double talk detectors, in order to substitute one of the DTDs for the other.
In general, the algorithms that have been proposed in the past for detecting double talk are relatively complex, insofar as they are based on analyzing the spectral envelope of the echo signal. Double talk detection is thus relatively demanding in terms of calculation power, and even then it does not provide a high degree of certainty about the presence of a genuine double talk situation.